Simply returns the value from 
species_params$constant_reproduction.
constantRDD(rdi, species_params, ...)Vector of density-independent reproduction rates \(R_{di}\) for all species.
A species parameter dataframe. Must contain a column
constant_reproduction.
Unused
Vector species_params$constant_reproduction
Other functions calculating density-dependent reproduction rate: 
BevertonHoltRDD(),
RickerRDD(),
SheperdRDD(),
constantEggRDI(),
noRDD()